As mentioned before; I have no idea as to what tire out there is providing the best performance for these side-by-sides.
I plan on upgrading to the 14 inch wheels and getting some tires but i dont know what i want. I would love to have something like these but in a 14 inch radial size: http://www.maxxis.com/products/atv/product_detail.asp?id=5445
but i don't think they will do well in the mud, although the RZR will be used mainly for trail riding and working around the farm. I want a stiff tire with a flat top  not balloon tires()
I have a buddy that put the Terracross on his Outlander (good looking tires). I have the Bighorns on my Ranger(love em). I have no complaints on the Bighorns although in the mud the Terracross did seem to shed the mud better. His Terracross are brand new so the wear and tear are yet to be seen. I will definatly consider the Terracross when I see how they hold up.
i really like my terracrosses. they are miles ahead of the pxt's. great on hard pack, made good traction in the snow, don't tear up my grass, seem to wear well and look a lot meaner!
Please give us a call at jagged X. There are tons of options for the new RZR. 12 inch and 14 inch chrome, black, polished. what ever you want tell us what type of riding you do and we can make some tire recomendations.